Together with our DNA results, we get a list of people, with whom we are related. Depending on the company, in which we got our test done these matches will be presented in two units – centimorgans (cM) and percents.
Centimorgan is a unit of measurement of shared DNA. Let’s think about it like a centimeter. Even the abbreviation of a centimorgan (cM) is very similar to a centimeter (cm), we just spell the centimorgan with the capital ‘M’.
Let’s think of our DNA as one long measuring tape, which is 7000 centimeters. 3500 centimeters of this tape we inherited from our mother and 3500 from our father.
In the same way, we think about centimorgans. The rule is, that the more centimorgans we share with someone, the closer we are related to them.
The total length of our chromosomes is around 7000 centimorgans https://genetics.thetech.org/ask-a-geneticist/centimorgans-vs-shared-segments (cM)
With one of our parents, we share around 3500 cM, with grandpa/grandma around 1750 cM, and with a second-degree cousin (shared grandparents) around 230 cM.
On the kinship table below we can see, how many cM (more or less) we share with our relatives. With the first-degree cousin, we share around 866 cM, with a second-degree cousin circa 229 cM. Based on how many centimorgans of shared DNA we have with somebody we can say what is our degree of kinship.
It happens, that our siblings will have a different number of shared DNA with other relatives than we do. It is because DNA is inherited randomly, so our siblings can share less or more of the DNA with the same relative.
The number of centimorgans will tell us, how much DNA we share with a relative. However, as you can see on the kinship table in centimorgans, many degrees of kinship have the same amount of shared DNA. On the infographic on the left, you can see the most probable possibilities for a match in the number of 850 cM.
This result matches at the same time:
- great grandfather (grey rectangle)
- great uncle (green)
- first degree cousin (yellow)
- son of a niece/nephew (purple).
How can we narrow down the degree of kinship? What will help us is the approximate age of the person, which will show us which relations we can rule out. For example, if the person is 40 and gets 850 cM with a person who is 80 years old, we can definitely rule out that it’s the child of a niece/nephew and most probably we can rule out them being a first-degree cousin. It’s a good idea to analyze the years, in which previous generations were born to check if there is no shift in generations (when in one family the children were born early and in the second late and f.ex. our uncle is our age).
The number of centimorgans, which we share with our matches helps us solve the mystery of how we may be related to someone. The more genetic relatives we contact and the more information we give them, the bigger the chance that we will get an answer and find a shared ancestor.
In the article ‘How to tell the degree of kinship based on shared DNA? Shared cM Project tool’ I will show you a helpful tool for ascertaining the degree of kinship.